1. Field of the Invention
This invention relates generally to determining geological properties of subsurface formations using MWD/LWD sensors, and particularly for improving the accuracy of signals from such sensors by restricting the tool motion during the measurements.
2. Description of the Related Art
A variety of techniques are utilized in determining the presence and estimation of quantities of hydrocarbons (oil and gas) in earth formations. These methods are designed to determine formation parameters, including among other things, the resistivity, porosity and permeability of the rock formation surrounding the wellbore drilled for recovering the hydrocarbons. Typically, the tools designed to provide the desired information are used to log the wellbore. Much of the logging is done after the well bores have been drilled. More recently, wellbores have been logged while drilling, which is referred to as measurement-while-drilling (MWD) or logging-while-drilling (LWD).
The various sensors utilized in the MWD/LWD environment are subjected to substantial motion and vibration that can compromise the quality of the resulting measurements. The MWD/LWD sensor sensitivity to motion can be roughly grouped in three categories: (i) sensors that are not significantly affected such as temperature sensors; (ii) sensors that can not tolerate substantially any motion such as formation pressure sampling systems; and (iii) sensors that produce degraded measurements such as, for example, Nuclear Magnetic Resonance (NMR) systems and other nuclear lithology sensors known in the art.
Nuclear Magnetic Resonance logging tools and methods are used for determining, among other things, porosity, hydrocarbon saturation and permeability of the rock formations. The NMR logging tools are utilized to excite the nuclei of the liquids in the geological formations surrounding the wellbore so that certain parameters such as nuclear spin density, longitudinal relaxation time (generally referred to in the art as T1) and transverse relaxation time (generally referred to as T2) of the geological formations can be measured. From such measurements, porosity, permeability and hydrocarbon saturation are determined, which provides valuable information about the make-up of the geological formations and the amount of extractable hydrocarbons.
The NMR tools generate a uniform or near uniform static magnetic field in a region of interest surrounding the wellbore. NMR is based on the fact that the nuclei of many elements have angular momentum (spin) and a magnetic moment. The nuclei have a characteristic Larmor resonant frequency related to the magnitude of the magnetic field in their locality. Over time the nuclear spins align themselves along an externally applied static magnetic field creating a net magnetization. This equilibrium situation can be disturbed by a pulse of an oscillating magnetic field, which tips the spins with resonant frequency within the bandwidth of the oscillating magnetic field away from the static field direction. After tipping, the spins precess around the static field at a particular frequency. At the same time, the magnetization returns to the equilibrium direction (i.e., aligned with the static field) according to a decay time known as the “spin-lattice relaxation time” or T1. For hydrogen nuclei a static field of 235 Gauss would produce a precession frequency of 1 MHz. T1 is controlled totally by the molecular environment and is typically ten to one thousand ms. in rocks.
Tool motion can seriously affect the performance of NMR tools used in an MWD/LWD environment because the measurement is not instantaneous and requires a non-varying magnetic field during the measurement time. NMR tools that have static magnetic fields and that have complete rotational symmetry are unaffected by rotation of the tool since the fields in the region of examination do not change during the measurement sequence. However, any radial or vertical component of tool motion will affect the NMR signal. As discussed in U.S. Pat. No. 5,705,927 issued to Kleinberg, resonance regions of many prior art instruments are of the order of 1 mm. Accordingly, a lateral vibration at a frequency of 50 Hz having an amplitude of 1 mm (100 g acceleration) would disable the instrument. The Kleinberg '927 patent discloses making the duration of each measuring sequence small, e.g. 10 ms, so that the drill collar cannot be displaced by a significant fraction of the vertical or radial extent of the sensitive region during a measurement cycle. However, using such short measurement times only gives an indication of the bound fluid volume and gives no indication of the total fluid volume.
There are numerous patents discussing the vibration of a rotating shaft subject to mechanical forces of the kind encountered by a drill string. U.S. Pat. No. 5,358,059 issued to Ho discloses the use of multiple sensors, including accelerometers, magnetometers, strain gauges and distance measuring sensors for determining the conditions of a drillstring in a borehole in the earth. The motion of the drill string in the borehole includes rotational motion, transverse (or radial) motion, and a whirl of the drill string. Whirling of the drillstring is the eccentric motion of the axis of the drillstring around the axis of the borehole and is a motion of great concern in NMR measurements. In an NMR tool, this motion causes the magnetic field strength in the region of examination to change with time, thereby degrading the measurement signal. Both whirl and various vibrational bending modes can cause radial motion that degrades the measurement.
The methods and apparatus of the present invention overcome the foregoing disadvantages of the prior art by providing a system for limiting the motion of the drill string in the region of measurement.